src/jsr166y/RecursiveAction.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package jsr166y;

/**
 * Recursive resultless ForkJoinTasks. This class establishes
 * conventions to parameterize resultless actions as <tt>Void</tt>
 * ForkJoinTasks. Because <tt>null</tt> is the only valid value of
 * <tt>Void</tt>, methods such as join always return <tt>null</tt>
 * upon completion.
 * 
 * <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that
 * sorts a given <tt>long[]</tt> array:
 *
 * <pre>
 * class SortTask extends RecursiveAction {
 *   final long[] array; final int lo; final int hi;
 *   SortTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo &lt; THRESHOLD)
 *       sequentiallySort(array, lo, hi);
 *     else {
 *       int mid = (lo + hi) &gt;&gt;&gt; 1;
 *       invokeAll(new SortTask(array, lo, mid),
 *                 new SortTask(array, mid, hi));
 *       merge(array, lo, hi);
 *     }
 *   }
 * }
 * </pre>
 *
 * You could then sort anArray by creating <tt>new SortTask(anArray, 0,
 * anArray.length-1) </tt> and invoking it in a ForkJoinPool.
 * As a more concrete simple example, the following task increments
 * each element of an array:
 * <pre>
 * class IncrementTask extends RecursiveAction {
 *   final long[] array; final int lo; final int hi;
 *   IncrementTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo &lt; THRESHOLD) {
 *       for (int i = lo; i &lt; hi; ++i)
 *         array[i]++;
 *     }
 *     else {
 *       int mid = (lo + hi) &gt;&gt;&gt; 1;
 *       invokeAll(new IncrementTask(array, lo, mid),
 *                 new IncrementTask(array, mid, hi));
 *     }
 *   }
 * }
 * </pre>
 *
 *
 * <p>The following example illustrates some refinements and idioms
 * that may lead to better performance: RecursiveActions need not be
 * fully recursive, so long as they maintain the basic
 * divide-and-conquer approach. Here is a class that sums the squares
 * of each element of a double array, by subdividing out only the
 * right-hand-sides of repeated divisions by two, and keeping track of
 * them with a chain of <tt>next</tt> references. It uses a dynamic
 * threshold based on method <tt>surplus</tt>, but counterbalances
 * potential excess partitioning by directly performing leaf actions
 * on unstolen tasks rather than further subdividing.
 *
 * <pre>
 * double sumOfSquares(ForkJoinPool pool, double[] array) {
 *   int n = array.length;
 *   int seqSize = 1 + n / (8 * pool.getParallelism());
 *   Applyer a = new Applyer(array, 0, n, seqSize, null);
 *   pool.invoke(a);
 *   return a.result;
 * }
 *
 * class Applyer extends RecursiveAction {
 *   final double[] array;
 *   final int lo, hi, seqSize;
 *   double result;
 *   Applyer next; // keeps track of right-hand-side tasks
 *   Applyer(double[] array, int lo, int hi, int seqSize, Applyer next) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *     this.seqSize = seqSize; this.next = next;
 *   }
 *
 *   double atLeaf(int l, int r) {
 *     double sum = 0;
 *     for (int i = l; i &lt; h; ++i) // perform leftmost base step
 *       sum += array[i] * array[i];
 *     return sum;
 *   }
 *
 *   protected void compute() {
 *     int l = lo;
 *     int h = hi;
 *     Applyer right = null;
 *     while (h - l &gt; 1 &amp;&amp;
 *        ForkJoinWorkerThread.getEstimatedSurplusTaskCount() &lt;= 3) {
 *        int mid = (l + h) &gt;&gt;&gt; 1;
 *        right = new Applyer(array, mid, h, seqSize, right);
 *        right.fork();
 *        h = mid;
 *     }
 *     double sum = atLeaf(l, h);
 *     while (right != null) {
 *        if (right.tryUnfork()) // directly calculate if not stolen
 *          sum += right.atLeaf(right.lo, right.hi);
 *       else {
 *          right.helpJoin();
 *          sum += right.result;
 *        }
 *        right = right.next;
 *      }
 *     result = sum;
 *   }
 * }
 * </pre>
 */
public abstract class RecursiveAction extends ForkJoinTask<Void> {

    /**
     * The main computation performed by this task. 
     */
    protected abstract void compute();

    /**
     * Always returns null
     */
    public final Void getRawResult() { return null; }

    /**
     * Requires null completion value.
     */
    protected final void setRawResult(Void mustBeNull) { }

    /**
     * Implements execution conventions for RecursiveActions
     */
    protected final boolean exec() {
        compute();
        return true;
    }

}
Revision:	1.1
Committed:	Tue Jan 6 14:30:31 2009 UTC (15 years, 4 months ago) by dl
Branch:	MAIN
Log Message:	Refactored and repackaged ForkJoin classes
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package jsr166y;
8
9	/**
10	* Recursive resultless ForkJoinTasks. This class establishes
11	* conventions to parameterize resultless actions as <tt>Void</tt>
12	* ForkJoinTasks. Because <tt>null</tt> is the only valid value of
13	* <tt>Void</tt>, methods such as join always return <tt>null</tt>
14	* upon completion.
15	*
16	* <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that
17	* sorts a given <tt>long[]</tt> array:
18	*
19	* <pre>
20	* class SortTask extends RecursiveAction {
21	* final long[] array; final int lo; final int hi;
22	* SortTask(long[] array, int lo, int hi) {
23	* this.array = array; this.lo = lo; this.hi = hi;
24	* }
25	* protected void compute() {
26	* if (hi - lo < THRESHOLD)
27	* sequentiallySort(array, lo, hi);
28	* else {
29	* int mid = (lo + hi) >>> 1;
30	* invokeAll(new SortTask(array, lo, mid),
31	* new SortTask(array, mid, hi));
32	* merge(array, lo, hi);
33	* }
34	* }
35	* }
36	* </pre>
37	*
38	* You could then sort anArray by creating <tt>new SortTask(anArray, 0,
39	* anArray.length-1) </tt> and invoking it in a ForkJoinPool.
40	* As a more concrete simple example, the following task increments
41	* each element of an array:
42	* <pre>
43	* class IncrementTask extends RecursiveAction {
44	* final long[] array; final int lo; final int hi;
45	* IncrementTask(long[] array, int lo, int hi) {
46	* this.array = array; this.lo = lo; this.hi = hi;
47	* }
48	* protected void compute() {
49	* if (hi - lo < THRESHOLD) {
50	* for (int i = lo; i < hi; ++i)
51	* array[i]++;
52	* }
53	* else {
54	* int mid = (lo + hi) >>> 1;
55	* invokeAll(new IncrementTask(array, lo, mid),
56	* new IncrementTask(array, mid, hi));
57	* }
58	* }
59	* }
60	* </pre>
61	*
62	*
63	* <p>The following example illustrates some refinements and idioms
64	* that may lead to better performance: RecursiveActions need not be
65	* fully recursive, so long as they maintain the basic
66	* divide-and-conquer approach. Here is a class that sums the squares
67	* of each element of a double array, by subdividing out only the
68	* right-hand-sides of repeated divisions by two, and keeping track of
69	* them with a chain of <tt>next</tt> references. It uses a dynamic
70	* threshold based on method <tt>surplus</tt>, but counterbalances
71	* potential excess partitioning by directly performing leaf actions
72	* on unstolen tasks rather than further subdividing.
73	*
74	* <pre>
75	* double sumOfSquares(ForkJoinPool pool, double[] array) {
76	* int n = array.length;
77	* int seqSize = 1 + n / (8 * pool.getParallelism());
78	* Applyer a = new Applyer(array, 0, n, seqSize, null);
79	* pool.invoke(a);
80	* return a.result;
81	* }
82	*
83	* class Applyer extends RecursiveAction {
84	* final double[] array;
85	* final int lo, hi, seqSize;
86	* double result;
87	* Applyer next; // keeps track of right-hand-side tasks
88	* Applyer(double[] array, int lo, int hi, int seqSize, Applyer next) {
89	* this.array = array; this.lo = lo; this.hi = hi;
90	* this.seqSize = seqSize; this.next = next;
91	* }
92	*
93	* double atLeaf(int l, int r) {
94	* double sum = 0;
95	* for (int i = l; i < h; ++i) // perform leftmost base step
96	* sum += array[i] * array[i];
97	* return sum;
98	* }
99	*
100	* protected void compute() {
101	* int l = lo;
102	* int h = hi;
103	* Applyer right = null;
104	* while (h - l > 1 &&
105	* ForkJoinWorkerThread.getEstimatedSurplusTaskCount() <= 3) {
106	* int mid = (l + h) >>> 1;
107	* right = new Applyer(array, mid, h, seqSize, right);
108	* right.fork();
109	* h = mid;
110	* }
111	* double sum = atLeaf(l, h);
112	* while (right != null) {
113	* if (right.tryUnfork()) // directly calculate if not stolen
114	* sum += right.atLeaf(right.lo, right.hi);
115	* else {
116	* right.helpJoin();
117	* sum += right.result;
118	* }
119	* right = right.next;
120	* }
121	* result = sum;
122	* }
123	* }
124	* </pre>
125	*/
126	public abstract class RecursiveAction extends ForkJoinTask<Void> {
127
128	/**
129	* The main computation performed by this task.
130	*/
131	protected abstract void compute();
132
133	/**
134	* Always returns null
135	*/
136	public final Void getRawResult() { return null; }
137
138	/**
139	* Requires null completion value.
140	*/
141	protected final void setRawResult(Void mustBeNull) { }
142
143	/**
144	* Implements execution conventions for RecursiveActions
145	*/
146	protected final boolean exec() {
147	compute();
148	return true;
149	}
150
151	}